Placoderms had heavy armour that covered their head
and thorax. The rest of the body was covered with small bony scales or
was without dental armour. Primitive bodies were covered with thick bony
ornamented platelets. On the more advanced bodies, the body scales were
reduced or absent, corresponding with the overall trend seen throughout
the evolution of these fish to reduce body weight. The head and trunk shields
of most placoderms were articulated by bony joints. This joint allowed
the forward part of the skull to tilt up, increasing the gap and allowing
for more mobility. Placoderms lacked teeth but biting or grinding structures
are often found in the dermal bones lining their mouths. They had one dorsal
fin, a pair of pectoral fins and a pair of pelvic fins with shark-like
shape and internal structure, many species also had a single anal fin.
These strange looking fish evolved into a variety of body forms in a relatively
short time. Many were torpedo-shaped except for the flattened Phyllolepidia
and the bottom-feeding Antiarch.